This invention relates to a raster output scanning (ROS) optical system, and, more particularly, to a binary diffractive optical element for controlling scanning beam intensity by providing uniform intensity in a raster output scanning (ROS) optical system.
Prior art printers and copiers utilizing a raster output scanning (ROS) optical system typically also utilize a rotating polygon having flat reflective surfaces, or facets, in parallel with the axis of rotation of the polygon. A beam, (or beams if a multiple diode is used) is emitted from a light source such as a helium-neon laser or a diode laser. The light is modulated according to an input electrical signal, directed through pre-polygon conditioning optics, and directed onto the facet surfaces of the rotating polygon. The beams are reflected from the facet surfaces through a post-polygon conditioning lens system and across the full process width of a photoreceptor medium.
The scanning beam reflected from the facet surfaces of the rotating polygon may have a decrease in intensity at both ends of the scan across the photoreceptor medium (a condition conventionally referred to as "frown").
Control over the light exposure level at the photoreceptor is required in all laser printers and copiers if acceptable prints and copies are to be produced. Indeed, beam intensity is critical if the proper exposure level for the particular photoreceptor used is to be assured. Variations in intensity across the scan line and from scan line to scan line, and in the laser output power, and in the transmittance, reflectance, and throughput efficiency of the various optical components must be compensated for. For example, a decrease in a spot intensity will result in a spot size decrease and will have a negative impact upon print quality especially in color systems.
And today, exposure control takes on added importance and criticality with the drive toward increased print resolution, half-toning, single-pass highlight color, and other developments where an intensity variation of no more than +/- 1% is desired.
Where gas lasers, such as a helium-neon laser, are used as the scanning beam source, light intensity is not directly variable at the source. In the past, if intensity control were to be provided, the drive power to the modulator was typically controlled. This allowed the diffraction efficiency of the modulator to be adjusted which, in turn, controlled the intensity of the scanning beam and provided the exposure levels desired. Today, however, the demand is for uniform exposure across the photoreceptor.
In many applications exposure control through adjustment of the modulator drive power is not a cost effective option.
Other possible sources of non-uniform intensity in the output scan are charge and development variations within a xerographic printing system; non-uniformities in polygon facet reflectivity, laser power degradation, loss of modulator efficiency and the like. Various techniques are known in the art for accomplishing some degree of compensation for these writing beam intensity variations. One technique incorporates the laser into a feed-back loop and then electronically controls the excitation level. Another technique describes a system for combining video image signals with beam intensity signals to provide an input to a modulator port which then regulates beam intensity. Still another technique is an intensity control device for a laser in a laser beam printer. The intensity control device stores a first representation of a present light intensity of the laser during its nonscanning mode and further stores a second representation of a user selected image density for a hard copy.
What is needed is a simple, inexpensive optical means for providing uniform intensity of a scanning beam along the scan line of a photoreceptor medium.
The propagation of a light beam can be changed by three basic means: reflection by a mirror, refraction by a lens and diffraction by a grating. Optical systems traditionally rely on reflection and refraction to achieve the desired optical transformation. Optical design, based on mirror and lens elements, is a well-established and refined process. Until recently, the problems with diffraction and fabricating high efficiency diffractive elements have made diffractive elements unfeasible components of optical systems.
The diffractive process does not simply redirect a light beam. Diffraction, unlike refraction and reflection, splits a light beam into many beams--each of which is redirected at a different angle or order. The percentage of the incident light redirected by the desired angle into some given diffraction order is referred to as the diffraction efficiency for that order. The diffraction efficiency of a diffractive element is determined by the element's surface profile.
Theoretically, on-axis diffractive phase elements consisting of a grating having a given period can achieve 100 percent diffraction efficiency. To achieve this efficiency, however, a continuous phase profile within any given period is necessary. The theoretical diffraction efficiency of this surface profile is also relatively sensitive to a change in wavelength. By contrast, refractive elements are relatively wavelength insensitive. The technology for producing high quality, high efficiency, continuous phase profiles of the diffraction does not presently exist.
A compromise that results in a relatively high diffraction efficiency and ease of fabrication is a multi-level phase grating. The larger the number of discrete phase levels, the better the approximation of the continuous phase function. The multi-level phase surface profiles of the grating can be fabricated using standard semiconductor integrated circuit fabrication techniques.
As disclosed in Binary Optics Technology: The Theory and Design of Multi-level Diffractive Optical Elements by G. J. Swanson of the Lincoln Laboratory at the Massachusetts Institute of Technology, (Technical Report 854, 14 Aug. 1989), herewithin incorporated by reference, and the resulting U.S. Pat. No. 4,895,790, a fabrication process starts with a mathematical phase description of a diffractive phase profile and results in a fabricated multi-level diffractive surface. The first step is to take the mathematical phase expression and generate from it a set of masks that contain the phase profile information. The second step is to transfer the phase profile information from the masks into the surface of the element specified by the lens design.
The first step involved in fabricating the multi-level element is to mathematically describe the ideal diffractive phase profile that is to be approximated in a multi-level fashion. The next step in the fabrication process is to create a set of lithographic masks which are produced by standard pattern generators used in the integrated circuit industry.
A substrate of the desired material, such as Ge, ZnSe, Si, GaAs, and SiO.sub.2, is coated with a thin layer of photoresist. A first lithographic mask is then placed in intimate contact with the substrate and illuminated from above with an ultraviolet exposure lamp. Alternately, pattern generators, either optical or electron beam, can expose the thin layer of photoresist. The photoresist is developed, washing away the exposed resist and leaving the binary grating pattern in the remaining photoresist. This photoresist will act as an etch stop.
The most reliable and accurate way to etch many optical materials is to use reactive ion etching. The process of reactive ion etching anisotropically etches material at very repeatable rates. The desired etch depth can be obtained very accurately. The anisotropic nature of the process assures a vertical etch, resulting in a true binary surface relief profile. Once the substrate has been reactively ion etched to the desired depth, the remaining photoresist is stripped away, leaving a binary surface relief phase grating.
The process may be repeated using a lithographic mask having half the period of the first mask. The binary phase element is recoated with photoresist and exposed using the second lithographic mask which has half the period of the first mask. After developing and washing away the exposed photoresist, the substrate is reactively ion etched to a depth half that of the first etch. Removal of the remaining photoresist results in a 4 level approximation to the desired profile. The process may be repeated a third and fourth time with lithographic masks having periods of one-quarter and one-eighth that of the first mask, and etching the substrates to depths of one-quarter and one-eighth that of the first etch. The successive etches result in elements having 8 and 16 phase levels. More masks than four might be used, however, fabrication errors tend to predominate as more masks are used.
This process produces a multilevel surface relief grating structure in the substrate. The result is a discrete, computer-generated structure approximating the original idealized diffractive surface. For each additional mask used in the fabrication process, the number of discrete phase levels is doubled, hence the name "binary" optical element or, more precisely, a binary diffractive optical element.
After only four processing iterations, a 16 phase level approximation to the continuous case can be obtained. The process can be carried out in parallel, producing many elements simultaneously, in a cost-effective manner.
A 16 phase level structure achieves 99 percent diffraction efficiency. The residual 1 percent of the light is diffracted into higher orders and manifests itself as scatter. In many optical systems, this is a tolerable amount of scatter. The fabrication of the 16 phase level structure is relatively efficient due to the fact that only four processing iterations are required to produce the element.
After the first etching step, the second and subsequent lithographic masks have to be accurately aligned to the existing pattern on the substrate. Alignment is accomplished using another tool standard to the integrated circuit industry, a mask aligner.
As noted, the photoresist on the substrate can be exposed with an electron-beam pattern generator. The e-beam direct-write process eliminates masks and their corresponding alignment and exposure problems. Binary optics have also been reproduced using epoxy casting, solgel casting, embossing, injection molding and holographic reproduction.
Binary optical elements have a number of advantages over conventional optics. Because they are computer-generated, these elements can perform more generalized wavefront shaping than conventional lenses or mirrors. Elements need only be mathematically defined: no reference surface is necessary. Therefore, wildly asymmetric binary optics are able to correct aberrations in complex optical systems, and elements can be made wavelength-sensitive for special laser systems.
The diffractive optical elements are generally thinner, lighter and can correct for many types of aberrations and distortions. It is possible to approximate a continuous phase profile with a stepwise profile of discrete phase levels.
It is an object of this invention to provide a simple, inexpensive optical means for providing uniform intensity of a scanning beam along the scan line of a photoreceptor medium.
It is an object of this invention to provide a binary diffractive optical element for controlling scanning beam intensity in a raster output scanning (ROS) optical system.